This invention relates to a cryopump and a method of operating the same, and more particularly, to a method and cryopump carring out a pumping operation while varying a pumping speed.
In, for example, U.S. Pat. No. 4,285,710, a cryopump of the aforementioned type is proposed wherein a two-stage expander comprises a first-stage expansion chamber and a second-stage expansion chamber. An aluminum support structure is attached to a distal end wall of the first-stage expansion chamber. A cylindrical pumping structure, constituting a first-stage pumping structure, is attached to the aluminum support structure so that the former is in an intimate thermal contact with the latter. A plate is attached to an end wall of the second-stage expansion chamber, and a frustoconical member, constituting a second-stage pumping structure, is attached to the plate so that the frustoconical member skirts a portion of the second-stage expansion chamber. A variable aperture flow restricting device is attached directly to a first (i.e. warmer) stage of a two-stage cryogenic pumping structure. Such a pumping apparatus is placed in a housing structure.
In this cryopump, the first-stage expansion of a helium gas occurs in the first-stage expansion chamber, so that the first-stage temperature is selected typically in the range of 50.degree.-80.degree. K. The second-stage expansion of the helium gas occurs in the second-stage expansion chamber, so that the second-stage temperature is selected typically in the range of 10.degree.-20.degree. K. The temperatures attainable in the first and second refrigeration stages are determined by the parameters of a refrigeration system. On the other hand, the temperature of the variable aperture flow restricting device, which requires a constant pumping speed, is maintained at such a cryogeneric temperature that is lower than the condensation temperature of gases (e.g. water vapor, carbon dioxide) and not lower than the condensation temperature of a selected gas (e.g. argon). The controlled variation at a pumping speed of this selected gas may be provided by throttling the apertured portion of the variable aperture flow restricting device.
The above proposed cryopump is used suitably in a sputtering system, and is capable of removing a gas, such as argon from a sputtering chamber as a constant water vapor pumping speed is maintained, at a different speed.
However, disadvantages of the above described cryopump resides in the fact that a cross-sectional area of a space through which a gas passes and the conductance decrease due to the variable aperture flow restricting device. Accordingly, a maximum pumping speed of the proposed cryopump decreases as compared with a cryopump which uses an expansion means having the same cold heat generating capacity, and no pumping speed varying functions. For example, if the variable aperture flow restricting device mentioned above is provided in a cryopump having no pumping speed varying functions, for example, a cryopump having a maximum pumping speed of 3000 l/s, the maximum pumping speed decreases to substantially 1500 l/s even when the throttling is done so as to obtain a maximum degree of opening of the variable aperture of this device. Namely, providing the variable aperture flow restricting device in a cryopump to furnish the pump with the pumping speed varying functions means reduces the maximum pumping speed thereof.
An object of the present invention is to provide a cryopump capable of varying a pumping speed by regulating, in accordance with ambient conditions, a temperature of a cold panel, and a method of operating the same.
The present invention enables a pumping speed of a cryopump, provided with a cryopanel cooled with the cold heat from a cold heat generating means to remove an ambient gas, and a means for regulating, in accordance with ambient temperature, the temperature of the cryopanel, to be varied by operating the cryopump while regulating the temperature of the cryopanel.